Allotropic Forms of Pure Iron – we know that pure substances may exist in more than one crystalline form. Each such crystalline form is stable over more or less well defined limits of temperature and pressure. This is known as allotropy or polymorphism.
The pure iron exists in the following three allotropic forms:
(a) Alpha iron which exists from the room temperature to 910° C. The alpha iron is ferromagnetic at room temperature. It has a body centred cubic (B.C.C.) structure.
(b) Gamma iron which exists between 910° C to 1404° C. It has a face centred cubic (F.C.C.) structure.
(c) Delta iron which exists between 1404° C to 1539° C (melting point of pure iron). It has a body centred cubic (B.C.C.) structure but has longer cube edge than B.C.C. structure of alpha iron.
